Aortic cannulas are used to return blood to the aorta while the heart is by-passed during heart surgery. These cannulas are purposely made with small diameters (typically six to eight millimeters, but even smaller for pediatric applications) to minimize the disruption to the aorta, which in many heart surgery patients have advanced complex atherosclerotic lesions with adherent blood thrombi. The flow velocities through these small diameter cannula must be very high in order to maintain a satisfactory blood flow rate of about five to seven liters per minute. In at least some styles of conventional aortic cannula now in use, this high velocity resulted in "jet" flow emanating from the distal end of the cannula, which acted as a nozzle. It is believed that the force of this narrow jet stream may dislodge atheromatous material and/or adherent thrombi from the walls of the aorta, causing embolisms. As surgical equipment and techniques improve, making heart surgery available to older and more seriously ill patients, thrombo-atheroembolisms affect an increasing number of patients due to the increasing extent of atherosclerosis with age.
The size of aortic cannula may be constrained by the constricted size of the aorta of the typical heart surgery patient. Moreover, the ability to diffuse flow is restricted by the fragility of the blood, which is easily damaged by the shear stresses associated with turbulence.
The aortic cannulas of the present invention are adapted to provide high volume flow at relatively lower flow velocities than the conventional aortic cannulas presently available, thereby reducing the jet flow and consequently reducing the incidence of thrombo-atheroembolisms. Generally aortic cannulas constructed according to the principles of this invention have a sidewall with a proximal end, a distal end, and a lumen therebetween for conducting blood. The cannula also has a distal end cap that blocks substantially all of the axial flow through the distal end of the cannula. There are a plurality of outlet openings in the sidewall of cannula, adjacent the distal end, to maintain flow volume of blood through the cannula. The distal end of the lumen is at least flush with the distal ends of the outlet openings, and preferably the lumen extends distally beyond the outlet openings.
The distal end of the lumen is configured to cause the flow through the outlet openings to extend more radially outwardly, with a concave, flat, or convex surface. In the preferred embodiment, the distal end of the lumen preferably has a centrally located land projecting proximally from the distal end. The surface of this land is generally a segment of a sphere. While it is preferable that the distal end of the lumen have this land, it is not essential, and desirable blood flow patterns without significant hemolysis can be achieved so long as the distal end of the lumen is at least flush with the distal ends of the outlet openings, but preferably distal end of the lumen extends between about 0.005 inches (0.127 mm) and about 0.200 inches (5.08 mm) distally beyond the distal end of the outlet openings, and more preferably about 0.090 inches (2.29 mm) beyond the distal end of the outlet openings.
The aortic cannula of the present invention creates a conical sheet flow, when viewed from the side, with a large cone angle such that the bulk of the flow from the cannula is more radial than axial. When viewed axially, the flow has a generally clover leaf shape, with a lobe corresponding to each outlet opening. This diffuse flow reduces the high velocity jetting that can occur with some aortic cannulas, while maintaining an adequate flow rate and minimizing damage to the blood.
These and other features and advantages will be in part apparent and in part pointed out hereinafter.